1. Field of the Invention
The present invention relates to a developing device and an image forming apparatus. More particularly, the present invention relates to a developing device with the use of a two-component developer including a toner and a magnetic carrier, and an image forming apparatus such as an electrostatic copying machine, a laser printer and a facsimile machine that form images using the developing device by an electrophotographic method.
2. Description of the Related Art
In the image forming apparatus using the electrophotographic method, an electrostatic latent image is formed on a surface of a photoconductor drum (toner image holder), toner is supplied to the photoconductor drum by means of a developing device to develop the electrostatic latent image, a toner image formed on the photoconductor drum through the development is transferred onto a sheet such as a paper sheet, and the toner image is fused onto the sheet by means of a fuser.
In recent years, for a full-color compliant and high-definition compliant image forming apparatus, a two-component developer (hereinafter, may be referred to simply as “developer”) has been widely used, which is excellent in toner charging stability. The developer includes a toner and a magnetic carrier. The toner and the carrier are agitated in a developing device to generate friction between the toner and the magnetic carrier, and the friction allows the toner to be appropriately charged.
The charged toner is supplied to a surface of a developer holder such as a developing roller. The toner supplied to the developing roller is transferred to an electrostatic latent image formed on a photoconductor drum by electrostatic attraction. Thus, a toner image based on the electrostatic latent image is formed on the photoconductor drum.
Further, such an image forming apparatus is required to be more high-speed and downsized. It is therefore necessary to quickly perform sufficient charging of a developer and to quickly convey the developer.
To this end, as a today's image forming apparatus, there has been proposed an image forming apparatus including a circulative developing device in order to instantly disperse supplementary toner into a developer to give an appropriate amount of charge.
For example, Japanese Unexamined Patent Publication No. 2004-272017 proposes a developing device comprising a developing container including a developing chamber and an agitating chamber that are separated by a partition wall to constitute a circulation route for a developer, the developing chamber comprising a developer carrying member for carrying the developer including a toner and a carrier and conveying the developer to an image bearing member, the agitating chamber comprising: a screw member having an agitating vane on a rotation axis and plate-like members (fins) attached to spaces in the agitating vane for agitating and conveying the developer; and a toner supply port for receiving the supplementary toner, wherein a second area spaced apart from the toner supply port by a predetermined distance and more toward the downstream side of the developer conveyance direction has more fins than a first area near the toner supply port.
When the screw member has a plurality of fins as in conventional cases, it is possible to enhance the miscibility between the developer and the supplementary toner, and enhance the agitating property and the charge impart ability of the developer thereby to prevent defective images. However, when the fins are provided, shearing stress (shear force) against the developer in a compressed state is increased and the stress on the developer during conveyance of the developer causes an external additive of the toner to be buried in the toner particles. In this case, the flow-ability of the developer is easily reduced and abrasion of the carrier is accelerated.